In data communication, a conventionally known packet transmission method is available for multiplexing and transmitting the data packets from a plurality of connections to a transmission link employed in common by the plurality of connections. Control is carried out as follows in a conventional packet transmission method. First, detection is performed to determine whether or not the transmission link has a vacant bandwidth. Next, if the transmission link has a vacant bandwidth, the packet is transmitted directly. If, however, the data link does not have a vacant bandwidth due to transmission of other packets with connections, packet transmission enters a waiting state until the transmission link becomes vacant. If the pattern for generating packets for each connection is random, and the connection number is sufficiently high, then the probability of a packet collision during multiplex transmission is averaged by this packet transmission method. Therefore, the average value is used for collision probability, and a transmission link that provides some degree of margin is prepared. In this way, a decline in service, such as increased delays, does not occur substantially, even if a transmission link for the transmission band that added the maximum rate which is offered to all the connections is not prepared.
Thus, it is possible to reduce costs since the transmission service provider can design facilities anticipating a statistical multiplexing effect. The following two points are prerequisites in a facilities design that anticipates such statistical multiplexing effect.    1) The delay condition of the target service shall be comparatively loose to an extent which does not impair the quality of service, even if the packets are made to wait for a moment in a packet multiplex transmitter.    2) The traffic characteristics of a packet which is supplied into the packet multiplex transmitter are average, and burst traffic does not occur.
However, data having strict delay conditions, or data that is cyclically generated is sometimes transmitted in data communication.
As an example of a strict delay condition, a service may be cited in which the delay of transmission data is acknowledged without modification by the user as a time lag, as in the case of image data in voice communication or in a video conference system.
Transmission of high efficiency coded data is available as an example of data having cyclical origin characteristics. The side which is transmitting this coded data converts the user's voice or moving image information into data of fixed or variable length by compressing. The transferred data is then transmitted cyclically or intermittently at a predetermined interval. The receiving side expands the user's voice or moving image information in its continuous, original form on the basis of the received coded data.
As another example of data having cyclical origin characteristics, there is communication through a radio link. In a radio transmission, radio frames are constituted cyclically in the radio link, and data is stored into a radio frame and transmitted. In this case, a communication from a user is carried out with this radio frame period. Therefore, the sending and receiving timing for data at the terminal of the radio link depends on the radio frame period.
For example, in the PDC system employed in digital mobile communication in Japan, the radio frame is formed at every 40 msec interval by combining 6 channels of time slots having a capacity of 280 bits. A super radio frame is formed by combining 18 of the radio frames. The super radio frame has a 720 msec period. In this radio frame design, two time slots are assigned to user communication in V-SELP (Vector Sum Excited Liner Prediction) voice data communication, while one time slot is assigned to user communication in PSI-CELP (Pith Synchronous Innovation CELP) voice data communication. Therefore, the voice communication data of these users usually originates at an integer timing of 20 msec or 40 msec.
As in the case of the examples mentioned above, data exists which has strict delay conditions, or which occurs cyclically. In particular, when a plurality of datum compete in a low speed transmission path, the amount of delay while awaiting transmission becomes serious. Thus, a facilities design which incorporates the statistical multiplexing effect described above cannot be applied to data communication of the aforementioned characteristics.
For the first example, an explanation will now be made of the case where ATM cell multiplex transmission is employed between a mobile communication base station and switching center, and cyclic data having strict delay conditions, such as voice data, is transmitted.
Mobile communication systems generally have extensive communication area. Thus, a mobile communication system arranges base stations that cover radio access to a region of a constant area. The mobile communication system connects these base stations and a switching center with a transmission link, and adopts a design in which the communication data from these base stations is exchanged and summarized. The transmission link speed between the base station and the switching center is influenced by the amount of traffic at the base station. However, in the case where voice communication predominates, the transmission link speed may become a low speed like 1.5 Mbps.
ATM cell multiplex transmission is sometimes applied to the transmission link between this base station and the switching center. In this case, the ATM cell multiplex transmission waiting delay in the ATM cell multiplex transmission portion has a large impact on the service quality as compared to a fixed network. The reason is as follows. First, ATM transmission in a fixed network sets 155 Mbps the basis. Because of this, in the ATM cell multiplex transmission portion, the transmission waiting time of one ATM cell is about 0.0027 msec. However, the delay becomes about 0.27 msec for transmitting the same single ATM cell in a 1.5 Mbps network. In other words, a delay quantity of about 100 times that of the fixed network occurs in the service because the multiplex transmission link is a low speed in mobile communication. In the case where N ATM cells compete in the ATM cell multiplex transmission portion, then a N-fold waiting time occurs. Thus, delay becomes an even more serious problem.
Therefore, when multiplex transmitting data having strict delay conditions at a specified quality by employing ATM cell multiplex transmission in the low-speed transmission link provided between the base station and the switching center, an approach is required which restricts the number of input connections, even if there is additional coverage in the band of the whole multiplex transmission path. As a result, a problem arises in that the statistical multiplexing cannot be sufficiently utilized.
The preceding example employed the case where data is multiplexed in ATM cell units. In contrast to this, a method is known for the efficient forwarding of data having a low transmission rate. In this method, the data is transmitted by multiplexing a AAL Type2 CPS Packet, in which a plurality of connections are mounted on one ATM cell, to one ATM virtual channel (VC). However, when this AAL Type2 CPS Packet is multiplexed to one VC of the ATM, a problem similar to that discussed in the preceding example occurs.
Next, the problem of the burst arrival of data in the terminal of the radio link will be discussed for the second example.
In a design in which ATM multiplex transmission is carried out between a mobile station and a base station which are connected by a radio link, a plurality of mobile stations transmit data to the base station. In this case, the timing for data receipt at the base station depends on the radio frame transmission timing between the mobile station and the base station. Therefore, in the case where each radio frame of each connection used by the plurality of mobile stations during communication is synchronously managed with the same timing, then the base station receives the data from all the mobile stations in a burst at the same timing. Therefore, the ATM cell also arrives at the base station's ATM multiplex transmitter in a burst at the same timing, so that the waiting delay described in the first example increases. Thus, in order to carry out multiplex transmission while satisfying quality, an approach is required in which the number of input connections is restricted, even if there is additional coverage in the band of the whole multiplex transmission path. As a result, a problem arises in that it is not possible to sufficiently utilize the statistical multiplexing effect.